JSTL, instanceof, and hasProperty

I’ve been doing a little bit of JSTL over the past week, especially custom tags. I’ve written custom tags in Grails before, and there you use actual Groovy code. I guess this was how custom tags used to be written (in Java), but now you can can build your own custom tags using the standard tag library. The standard tag library is still pretty useful when it comes to building custom tags. Since it’s not straight Java, it forces you think really hard about your logic. You don’t want to put any business or application logic in your tag, and you want to restrict everything to view or presentation logic. A side effect of it not being Java is that if you want to do anything extremely complicated, you’re probably better off writing the tag in Java (making sure that you don’t let any business logic creep in).

While writing my own custom tag, I noticed that although instanceof is a reserved word in the JSTL EL (expression language), it is not supported as an operator. The reason I wanted to use the instanceof operator is that I have an attribute that could either be a List or a Map and depending on the type, I wanted to do different things.

Another thing I was trying to do, was to inspect the incoming object to see if it had a certain property (reflection). JSTL uses reflection so that you can access the properties of an object via dot notation, if they follow the JavaBean naming-convention. However, there was no way for me to see if an object had a certain property. To solve both these problems, I wrote my own JSTL functions.
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